Abstract
This doctoral dissertation discusses the sedimentology and dynamics of selected, modern and ancient clastic depositional systems (alluvial fans and colluvial aprons) at continental basin margins. The focus on single depositional systems gave the opportunity to devote particular attention to sedimentary processes and interactions on the basis of detailed field observations.
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The recent emphasis on modelling, remote sensing and basin-scale analysis in sedimentary geology provides large amounts of raw and conceptual data, but to the cost of gradually losing touch with the complexity of real natural processes. In view of the many long-standing misconceptions and generalizations in the literature on terrestrial proximal depositional systems, the work presented herein demonstrates that a field-based approach is the most reliable answer to still open questions. The Neogene intermontane Teruel Basin (central Spain) was chosen to investigate ancient alluvial-fan successions because of its exceptional outcrops, the complete preservation of sedimentary events in an endorheic setting, and the high-resolution chronologies and paleoclimatic context available from previous cyclostratigraphic research on mudflat and ephemeral lacustrine deposits. Detailed correlations between these and alluvial-fan strata sourced by the adjacent basin margin show that orbitally forced climate change could regulate sedimentation not only in low-energy basinal settings, but also in a high-energy, high-gradient alluvial fan. Phase relationships between interfingering fan-mudflat strata and insolation curves demonstrate that, during the last stages of fan development, debris-flow events with the highest sediment loads took place during transitions from relatively arid to humid climate. Such events were probably related to adjustments in catchment hydrology and sediment yield under increasing seasonality and precipitation, which destabilized highland slopes but before the widespread establishment of a vegetation mantle. However, facies analysis of the entire extent of the fan succession shows a longer-term control of basement geology on processes and architecture. In particular, the availability of carbonate versus clay-rich clastic source rocks in the catchment was fundamental in determining the dominance of hyperconcentrated sheetfloods or cohesive debris-flows during different phases of fan aggradation. Analysis of an extensive colluvial depositional system along the coast of the Atacama Desert (northern Chile) shows an even stronger dependence of processes and depositional architectures on the morphology and geographic orientation of different piedmont sectors. However, the strike-oriented variability in the relative importance of different mass-flow processes and the influence of aeolian deposition over active slope surfaces are not always perfectly reflected in the corresponding stratigraphic products, because the preservation of local colluvium is strongly tied to the surface hydrology of the slopes, which also varies with piedmont morphology. General stratigraphic models of proximal deposystems in terrestrial settings have not yet been satisfactorily developed, probably owing to the extraordinary spatial and especially temporal variability of basement and catchment controls exerted on sedimentary processes and architectural evolution. Nonetheless, it is clear that prediction of subsurface stratigraphic patterns in the rock record, and geohazard prediction in modern settings, can be significantly enhanced by careful consideration of the geologic context in which these depositional systems form.
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